The invention relates to a sensor for detecting the concentration of sulphur compounds in a liquid.
The main emitters of nitrogen oxides (NOx) in industrial countries are transport, power stations fired by fossil fuels and industrial plants. While the emissions from power stations and industry are continuing to decrease, the proportion due to traffic is becoming ever more prominent.
The NOx emissions of petrol-powered 4-stroke engines can be drastically reduced by operation at an air ratio xcex=1 and exhaust gas purification after the engine by a 3-way catalyst. In principle, this possibility does not exist in a mixture-regulated diesel engine which is operated superstoichometrically. Due to the high proportion of oxygen in the exhaust gas, no catalyst which can reduce the NOx emissions without the addition of reducing agents, e.g. hydrocarbons or ammonia-forming compounds, has been found.
The same applies to 4-stroke engines which are run lean. In that case, there have for some time been vehicles having an exhaust gas train provided with a catalyst which is run lean (xcex greater than 1) for a certain time and can store nitrogen oxides during this time. After this xe2x80x9cstorage phasexe2x80x9d in which the catalyst is xe2x80x9cchargedxe2x80x9d with the nitrogen oxides to be stored, there follows a generally much shorter desorption phase during which the catalyst is xe2x80x9cemptiedxe2x80x9d. During the desorption phase, the engine is run rich (xcex less than 1). Strehlau W., Leyer J., Lox E. S., Kreuzer T., Hori M., Hoffmann M.: New developments in lean NOx catalysis for gasoline fueled passenger cars in Europe, SAE-Paper 962047 (1996) describes a catalyst which is suitable for such conditions. The expression xe2x80x9cNOx trapxe2x80x9d is also used in the literature.
The catalyst materials used for an NOx trap include, inter alia, alkali metal compounds or alkaline earth metal compounds which are responsible for storage of the nitrogen oxides. Unfortunately, such compounds react preferentially with sulphur oxides which are likewise present in the exhaust gas and are formed during combustion of the sulphur compounds (e.g., mercaptans, thiophenes, thioethers, thioesters, disulphides) present in the fuel and are converted into alkali metal or alkaline earth metal sulphates. For this reason, such storage catalysts lose their storage capability after a certain operating time which is dependant on the concentration of the sulphur compounds in the fuel. They have to be desulphated. Methods for desulphurising such storage catalysts are described, for example, in European Published Patent Application No. 0 858 837, European Published Patent Application No. 0 860 595, and European Published Patent Application No. 0 899 430. The engines are no longer run during the regeneration phase in the normal alternating lean/rich operation described above but are instead continually run rich and a certain minimum temperature required for desulphurisation has to be reached.
Since fuel consumption is significantly increased during such a desulphurisation phase because of running rich, desulphurisation should be carried out only when absolutely necessary. The optimum time for a desulphurisation phase can be determined as described in European Published Patent Application No. 0 860 595 by a functional relationship depending on the content of sulphur compounds in the fuel used, the current mass flow of fuel in the internal combustion engine and the current exhaust gas temperature at the xe2x80x9cNOx trapxe2x80x9d, but the content of sulphur compounds in the fuel is usually unknown. However, there is at present no known sensor which would be suitable for determining the content of sulphur compounds in a fuel in a production vehicle.
To protect the NOx trap from sulphur compounds, German Published Patent Application No. 198 13 654 describes the installation of a construction containing a sulphur-storing compound upstream of the NOx trap so that all sulphur remains in the SOx trap and the function of the NOx trap is therefore not impaired. However, the SOx trap also has to be regenerated from time to time or, if its storage capacity is sufficiently large, replaced. Determination of the xe2x80x9cdegree of loadingxe2x80x9d of such an SOx trap requires a sulphur sensor so that the degree of loading can be derived from the values measured by the sensor using an integration method.
The degree of loading of a desulphurisation device for fuel, as described in German Published Patent Application No. 198 45 397, could also be determined using a sulphur sensor and an integration method.
The concentration of sulphur compounds in the fuel can be of interest not only in the case of a 4-stroke engine but also in the case of a diesel engine. This is particularly true when particular novel sulphursensitive NOx- or NH3-storing catalysts are to be used for exhaust gas purification.
The concentration of sulphur compounds in methanol, which has been discussed as fuel for fuel cell vehicles, can also be of interest.
The concentration of sulphur compounds in other fuels such as ethanol or liquefied gas can also be of interest.
In addition, the concentration of sulphur compounds in fuel oil used for firing stationary facilities may be of interest.
It is an object of the present invention to provide a sensor for determining the concentration of sulphur compounds in a liquid, in particular a motor fuel, which is inexpensive to produce and has a small size so that it is suitable for use in production vehicles.
The sensor according to the present invention for determining the concentration of sulphur compounds in a liquid includes:
a working electrode in contact with the liquid to be analyzed;
a reference electrode insulated from the liquid to be analyzed;
a liquid-impermeable membrane which is located between the working electrode and the reference electrode and is permeable to an ion which may form a chemical compound with the sulphur compounds in the liquid to be analyzed; and
a reference material which is in contact with the reference electrode or forms the latter.
The sulphur sensor according to the present invention is inexpensive to produce and is, due to its small size and lack of complexity, particularly suitable for use in a production vehicle.
It may be used, in particular, for determining the degree of sulphur charge of an NOx trap in the exhaust gas purification system of a 4-stroke engine which is run lean. All other applications described above are also possible for the sensor of the invention.